Motor control circuit with power factor correction

ABSTRACT

A motor control circuit with power factor correction capabilities that optimizes the voltage and current load applied to an electric motor for different motor speeds and torque levels. The preferred motor control circuit includes a power factor correction circuit and a step down conversion circuit through which current passes before it reaches the motor. A microprocessor preferably monitors the current supplied to the motor and the motor&#39;s speed. If the microprocessor determines that the current supplied to the motor is too high, it can reduce the level of current by either using a pulse width modulation (PWM) digital-to-analog control circuit to instruct the power factor correction circuit to reduce current or it can use a PWM digital control circuit to instruct the step down conversion circuit to reduce current. An output voltage limiter circuit can be used to detect the voltage of current supplied to the motor and turn off current to the motor if the voltage is above a predetermined level.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. § 119(e)to U.S. Provisional Application No. 62/802,667, filed on Feb. 7, 2019,and entitled “Simplified Motor Control Circuit With Power FactorCorrection,” the disclosure of which is hereby incorporated by referencein its entirety.

FIELD OF THE INVENTION

The present invention relates to a motor control circuit that performspower factor correction and helps optimize the electrical output load tothe motor, particularly for low speed, low torque motor operation.

BACKGROUND OF THE INVENTION

Recently, governments around the world have encouraged efficient energyuse for electronic products and home appliances. In response to thisinitiative, power factor correction (PFC) circuits have been developedfor such electronic products and home appliances. A PFC circuit is acircuit for switching input power and adjusting a phase difference(power factor) between a current and a voltage of the input power toefficiently transfer power to a load.

PFC circuits have been the subject of numerous prior art patents,including Nanos's U.S. Pat. No. 5,561,356; Ramu's U.S. Pat. No.8,264,182; Park's U.S. Pat. No. 8,766,581; Heo's U.S. Pat. No.9,148,062; Kim's U.S. Pat. No. 9,172,323 and Jeong's U.S. Pat. No.9,331,598.

Integrated circuit chips have been created to allow circuit designers toinclude PFC functionality in their circuits. One such PFC chip is the“FL7701” Smart LED Lamp Driver IC with PFC Function chip manufacturedand sold by On Semiconductor (Linear Technologies). While the FL7701chip is useful for adding PFC functionality to an electrical circuit, itwas designed for continuous resistive load LED lamp applications.Nonetheless, there are many other applications besides LED lamps wherePFC functionality is needed, especially for inductive loads such asmotors used in electronic products and home appliances.

Electric motors present circuit designers with different requirementsand challenges than LED lamps, namely the capacitive and inductivenature of the motor load. For example, electric motors can be operatedat different speeds and torque levels. While the FL7701 chip isparticularly useful for LED lamps, it is not optimized for operationwith the full range of speeds and torques utilized by electric motors.The FL7701 chip is particularly problematical when a motor is operatedat low speed and low torque. For low speed, low torque operation, thevoltage and current supplied to the motor by the FL7701 chip can be toohigh. What is needed is an improved PFC circuit that safely enables andoptimizes the full range of motor capabilities, particularly for lowspeed, low torque operation.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to a motor control circuit with PFCcapabilities that optimizes the voltage and current load applied to theelectric motor for different motor speeds and torque levels. In thepreferred embodiment, the motor control circuit includes a chipproviding PFC and current control functionality. The output of the PFCchip is connected to a step down conversion circuit to drive the motor.

To optimize motor performance, additional circuitry is preferablyconnected to the electric motor, PFC chip and step down conversioncircuit. For example, the current load and speed of the motor ispreferably monitored by a firmware microprocessor. When appropriate, themicroprocessor provides a buffered motor drive signal to a pulse widthmodulation (PWM) digital-to-analog circuit connected to the PFC chip.This PWM digital-to-analog circuit adjusts the output current of the PFCchip to optimize the current level supplied to the motor. Through use ofan optocoupler, the PWM digital-to-analog circuit also preferablyisolates the firmware microprocessor from high voltages in the motordrive circuit.

Additionally, current supplied to the motor can be optimized by a PWMdigital control circuit which receives a buffered motor drive controlsignal from the microprocessor. Where the current supplied to the motoris too high, this PWM digital control circuit can intercept and overridethe output signal from the PFC chip at the step down conversion circuit.The PWM digital control circuit is particularly useful when the outputload from the PFC chip is set at its minimum level but the powersupplied to the motor is still too high. By adjusting the duty cycle,this PWM digital control circuit can make the output power very low orturn it off completely.

Finally, the output voltage across the motor is monitored by an outputvoltage limiter circuit which is capable of limiting the speed of themotor. This output voltage limiter circuit preferably includes a Zenerdiode and an optocoupler. When the output voltage limiter circuitdetermines that the output voltage is too high, such as higher than120V, the Zener diode and optocoupler in the voltage limiter circuitdirects the PFC chip to pull down the output current. This currentrestriction, which results in a voltage and corresponding speedreduction to the motor, is accomplished by circuit hardware which isindependent of the microprocessor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified functional block diagram of a preferredembodiment of the present invention.

FIG. 2 is a schematic circuit diagram of a preferred embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, a preferred form of motor control circuit 100of the present invention is shown in functional block diagram form. Themotor control circuit 100 preferably includes a motor 200, a PFC circuit300, a step down conversion circuit 400, a PWM digital-to-analog circuit500, a PWM digital control circuit 600, a firmware microprocessor 700and an output voltage limiter circuit 800. The motor 200 is an electricmotor, preferably a Permanent Magnet D.C., of the type commonly used inconsumer electric products and home appliances. Nonetheless, theprinciples of this invention are also applicable to other types ofelectric motors.

Turning now to FIG. 2, the PFC circuit 300 may correct a power factor byswitching rectified power obtained from alternating current (AC) powerthrough a bridge diode 12. The PFC circuit 300 of the present inventionpreferably includes a PFC chip 302, such as the FL7701 chip manufacturedand sold by On Semiconductor (Linear Technologies). The preferred FL7701chip has eight pins, labeled CS, OUT, VCC, RT, HV, NC, GND and ADIM.System reference voltage from the bridge diode 12 is provided to the PFCchip 302 through the HV pin. Once the PFC chip 302 has performed itspower factor correction function, a transistor control current leavesthe PFC chip 302 though its OUT pin and controls the power transistor402. The primary flow of current is through the bridge rectifier 12 andthe inductor 408 before reaching the motor 200. The power transistor 402is the final piece in the current flow path back to the low side of themain power supply (the AC line input depicted to the left of the bridgerectifier 12).

The power transistor 402 in step down conversion circuit 400 is theprimary control element responsible for controlling current flow fromits input (supply) to its output (load). In the preferred embodiment,the step down conversion circuit 400 preferably includes a powertransistor 402 (which is preferably an insulated-gate bipolar transistor(IGBT)), a diode 404, a capacitor 406 and an SMC compliant inductor 408.The power transistor 402 is a three-terminal power semiconductor deviceused as an electronic switch. Since it is designed to turn on and offrapidly, the power transistor 402 can synthesize complex waveforms withpulse width modulation (PWM).

PWM is a method of reducing the average power delivered by an electricalsignal by effectively chopping it up into discrete parts. The averagevalue of voltage (and current) fed to the load is controlled by turningthe switch on and off at a fast rate between supply and load. The longerthe switch is on compared to the off periods, the higher the total powersupplied to the load. PWM is particularly suited for running inertialloads such as electric motors, which are not easily affected by discreteswitching because they have the inertia to react slowly. Nonetheless,the PWM switching frequency needs to be high enough not to affect theload, which is to say that the resultant waveform perceived by the loadmust be as smooth as possible.

PWM digital-to-analog circuit 500 is one of the preferred ways thepresent invention varies the current load applied to the motor 200 sothat it is optimized for different motor speeds and torque. The PWMdigital-to-analog circuit 500 is particularly useful when the motor 200needs to be operated at a low-speed, low-torque level where the voltageand/or current supplied to the motor 200 by the PFC chip 302, even atits minimum level, is too high. To operate the PWM digital-to-analogcircuit, signals 202, 204 are received from the motor 200 by themicroprocessor 700 indicative of the operating speed 202 of the motor200 and current 204 being supplied to the motor (FIG. 1). Based uponthese signals 202, 204, the microprocessor 700 determines whether thecurrent needs to be adjusted to achieve optimum motor performance. Ifadjustment is needed, the microprocessor 700 can send a buffered controlsignal 702 to the PWM digital-to-analog control circuit 500 to adjustthe output of the PFC chip 302.

The buffered control signal 702 passes through resistor 502 until itreaches optocoupler 504 which, in the preferred embodiment, is acombination of LED-photodiode 506 and phototransistor 508. Theoptocoupler 504 transfers the buffered control signal 702 using light.Voltage to reconstitute the buffered control signal 702 is provided bythe VCC pin of PFC chip 302 and passes through capacitors 510, 512 and514. The reconstituted control signal 706 passes through resistors 516,518 until it reaches the ADIM input pin of PFC chip 302. If themicroprocessor 700 determines that the current being supplied to themotor 200 is too high, the reconstituted control signal 706 will directthe PFC chip 302 to intermittently turn off current to the motor 200using pulse width modulation . Conversely, if the microprocessor 700determines that the current supplied to the motor 200 is too low, thereconstituted control signal 706 can direct the PFC chip 302 to increasethe current provided to the motor 200.

A second preferred way in the present invention to vary current loadapplied to the motor 200 so that it is optimized for different motorspeeds and torque is through the use of PWM digital control circuit 600.If the microprocessor 700 determines that current needs to be adjustedto achieve optimum motor performance, particularly if the motorfrequency is determined to be too high, the microprocessor 700 can senda buffered control signal 704 to PWM digital control circuit 600. Thebuffered control signal 704 passes through resistor 602 until it reachesoptocoupler 604 which, in the preferred embodiment, is a combination ofLED-photodiode 606 and phototransistor 608. Again, the optocoupler 610uses light to transfer the buffered control signal 704 and protectcontrol circuitry from high voltages. Voltage to reconstitute thebuffered control signal 704 is provided by VCC pin of PFC chip 302 andpasses through capacitor 514. The reconstituted control signal 604 isthen fed to the power transistor 402 of step down conversion circuit400. When activated, the PWM digital control circuit 600 can interceptand override the output signal from the PFC chip 302 at the step downconversion circuit 400 and, in the process, intermittently turn off thepower transistor 402 if the motor frequency is too high. The PWM digitalcontrol circuit 600 is particularly useful when the output load from thePFC chip 302 is set at its minimum level but the power supplied to themotor 200 is still too high. By adjusting the duty cycle, this PWMdigital control circuit 600 can make the output power very low or turnit off completely.

Output voltage limiter circuit 800 provides a third way of optimizingmotor performance. The output voltage limiter circuit 800 allows currentto the motor 200 to be turned off if voltage across the motor 200 ishigher than a predetermined level, such as 120V. The output voltagelimiter circuit 800 preferably includes diode 806, Zener diode 808,resistor 810 and optocoupler 804.

A Zener diode is a diode that normally allows current to flow in theconventional manner from its anode to its cathode. Nonetheless, when thevoltage across the Zener diode reaches a predetermined level, referredto as the “Zener voltage,” the junction will break down and current willflow in the reverse direction.

For the output limiter circuit 800 of the present invention, currentnormally flows from bridge diode 12 through SMC compliant inductor 408and into motor 200. The Zener diode 808 normally blocks current fromflowing through the output voltage limiter circuit 800. Nonetheless,when the voltage reaches a predetermined threshold level, such as 120V,the Zener diode junction will break down so that current flows throughthe Zener diode 808 of the output limiter circuit 800. The predeterminedthreshold voltage for the Zener diode 808 is preferably set at a levelwhere the current and voltage impairs operation of the motor 200. Whenthat predetermined threshold level is reached, current flows throughresistor 810, Zener diode 808 and diode 806. In this mode of operation,Zener diode 808 works with optocoupler 804 to send a signal to the ADIMpin of PFC chip 302 to temporarily turn off current to the motor 200until the voltage across the Zener diode 808 has dropped below thepredetermined threshold level. In this way, the output limiter circuit800 protects the motor 200 from potentially damaging high levels ofvoltage and current.

In the foregoing specification, the invention has been described withreference to specific preferred embodiments and methods. It will,however, be evident to those of skill in the art that variousmodifications and changes may be made without departing from the broaderscope of the invention as set forth in the appended claims. Thespecification and drawings are, accordingly, to be regarded in anillustrative, rather than restrictive sense; the invention being limitedonly by the appended claims.

What is claimed is:
 1. A control circuit for optimizing the operation ofan electric motor comprising: an electric motor; a power factorcorrection circuit: and, a step down conversion circuit, wherein currentto said electric motor passes through said power factor correctioncircuit and said step down conversion circuit before reaching saidmotor; further comprising a microprocessor which receives signalsindicative of the current supplied to said motor and, based upon saidsignals, said microprocessor can reduce the current supplied to saidmotor using a pulse width modulation (PWM) digital-to-analog controlcircuit.
 2. The control circuit of claim 1 wherein said power factorcorrection circuit is a power factor correction chip.
 3. The controlcircuit of claim 1 wherein said PWM digital-to-analog control circuitreduces current to said motor through signals it provides to said powerfactor correction circuit.
 4. The control circuit of claim 3 whereinsaid PWM digital-to-analog control circuit reduces current to said motorby directing said power factor correction circuit to intermittently turnoff power to said motor through pulse width modification.
 5. The controlcircuit of claim 1 wherein said PWM digital-to-analog control circuitincludes an optocoupler.
 6. The control circuit of claim 1 wherein saidstep down power conversion circuit includes an insulated-gate bipolartransistor (IGBT).
 7. A control circuit for optimizing the operation ofan electric motor comprising: an electric motor; a power factorcorrection circuit: and, a step down conversion circuit, wherein currentto said electric motor passes through said power factor correctioncircuit and said step down converter circuit before reaching said motor;further comprising a microprocessor which receives signals indicative ofthe current supplied to said motor and, based upon said signals, saidmicroprocessor can reduce the current supplied to said motor using apulse width modulation (PWM) digital control circuit.
 8. The controlcircuit of claim 7 wherein said PWM digital control circuit reducescurrent to said motor through signals it provides to said step downconversion circuit.
 9. The control circuit of claim 8 wherein said PWMdigital control circuit reduces current to said motor by directing saidstep down conversion circuit to intermittently turn off power to saidmotor through pulse width modification.
 10. The control circuit of claim7 wherein said step down power conversion circuit includes aninsulated-gate bipolar transistor (IGBT).
 11. The control circuit ofclaim 7 wherein said PWM digital control circuit includes anoptocoupler.
 12. A control circuit for optimizing the operation of anelectric motor comprising: an electric motor; a power factor correctioncircuit: and, a step down conversion circuit, wherein current to saidelectric motor passes through said power factor correction circuit andsaid step down conversion circuit before reaching said motor; furthercomprising an output voltage limiter circuit capable of detecting thevoltage of current supplied to said motor and having current to saidmotor turned off if said voltage is above a predetermined level.
 13. Thecontrol circuit of claim 12 wherein said output voltage limiter circuitincludes a Zener diode.
 14. The control circuit of claim 13 wherein saidZener diode will cause a signal to be sent to said power factorcorrection circuit to turn off current to said motor if the detectedvoltage of current supplied to said motor is above a predeterminedlevel.
 15. The control circuit of claim 12 wherein said output voltagelimiter circuit includes an optocoupler.
 16. The control circuit ofclaim 12 wherein said predetermined voltage level is 120V.
 17. Thecontrol circuit of claim 12 wherein said output voltage limiter circuitcauses current to said motor to be turned off intermittently if saidvoltage is above a predetermined level.
 18. A method of optimizing theoperation of an electric motor comprising the steps of: supplyingcurrent to an electric motor by first passing said current through apower factor correction circuit and a step down conversion circuitbefore it reaches said motor; transmitting signals indicative of currentsupplied to said motor to a microprocessor; using said microprocessor todetermine whether the current supplied to said motor needs to bereduced; transmitting signals from said microprocessor to a pulse widthmodulation (PWM) digital-to-analog control circuit to reduce current tosaid electric motor if said microprocessor determines that said existingcurrent level is too high.
 19. The method of claim 18 wherein said PWMdigital-to-analog control circuit reduces current to said motor throughsignals it provides to said power factor correction circuit.
 20. Themethod of claim 19 where said PWM digital-to-analog control circuitreduces current to said motor by directing said power factor correctioncircuit to intermittently turn off power to said motor through pulsewidth modulation.
 21. A method of optimizing the operation of anelectric motor comprising the steps of: supplying current to an electricmotor by first passing said current through a power factor correctioncircuit and a step down conversion circuit before it reaches said motor;transmitting signals indicative of current supplied to said motor to amicroprocessor; using said microprocessor to determine whether thecurrent supplied to said motor needs to be reduced; transmitting signalsfrom said microprocessor to a pulse width modulation (PWM) digitalcontrol circuit to reduce current to said electric motor if saidmicroprocessor determines that said existing current level is too high.22. The control circuit of claim 21 wherein said PWM digital controlcircuit reduces current to said motor through signals it provides tosaid step down conversion circuit.
 23. The control circuit of claim 22wherein said PWM digital control circuit reduces current to said motorby directing said step down conversion circuit to intermittently turnoff power to said motor through pulse width modulation.
 24. The controlcircuit of claim 21 wherein said step down power conversion circuitincludes an insulated-gate bipolar transistor (IGBT).
 25. A method ofoptimizing the operation of an electric motor comprising the steps of:supplying current to an electric motor by first passing said currentthrough a power factor correction circuit and a step down conversioncircuit before it reaches said motor; using an output voltage limitercircuit to detect the voltage of current supplied to said motor; and,using said output voltage limiter circuit turn off current to said motorif said voltage is determined to be above a predetermined level.
 26. Themethod of claim 25 wherein said output voltage limiter circuit includesa Zener diode.
 27. The method of claim 26 wherein said Zener diode willcause a signal to be sent to said power factor correction circuit toturn off current to said motor if the detected voltage of currentsupplied to said motor is above a predetermined level.
 28. The method ofclaim 27 wherein said predetermined voltage level is 120V.